After defining your stair in the position and geometry it should be in your project, activate the Set Stair Boundary Conditions command in the Stair auxiliary toolbar.
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The Set Boundary Conditions dialog will open.
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After marking the solution with building system title
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, make your arrangement so that only the option/options that are supported on each side of the staircase will remain active.
If you want to solve the staircase element with the structure, it would be a better solution to select the Semi-rigid diaphragm option from the Analysis Settings/General tab.
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